1. Field of the Invention
This invention relates to a system for controlling the drive joints in a robot, more particularly to a system for controlling drive joints in a legged mobile robot such as a biped walking robot so as to soften footfall impact.
2. Description of the Prior Art
As shown in FIGS. 19A and 19B, so long as the feet of an autonomous biped walking robot or other type legged mobile robot are in contact with the ground, the robot is acted on by forces from the ground surface. Such an external force will hereinafter be referred to as a "ground reaction force." One of the ground reaction forces received by a legged mobile robot during locomotion acts as a vibration-producing impact over a short period following the landing (footfall) of the foot of a free leg. This will hereafter be referred to as a "footfall impact." A large footfall impact may act as a disturbance which destabilizes the robot's locomotion. In the worse case it may cause the robot to tip over.
It was for this reason that, earlier, in Japanese Patent Application No. 1(1989)-297,199 (Japanese Laid-Open Patent Publication No. 3(1991)-161,290), the assignee proposed a technology for softening footfall impact by controlling the ankle drive joints in response to footfall impact. If the capability of absorbing and softening footfall impact can be enhanced still further, there can be expected to be obtained even more stable robot locomotion. In particular, there can be expected to be obtained more stable locomotion in situations where the footfall impact tends to be high, as during walking over rough terrain or walking at high speed.
Footfall impact can be treated as a force acting on the robot's center of gravity. As shown in FIG. 20, however, the earlier technology softened footfall impact solely through control of the ankle joints (indicated by hatching). Since it was therefore unable to achieve large shifts in the center of gravity, the absorption of footfall impact was inadequate. Another problem with the earlier technology is that it cannot perform an absorption operation when the whole sole of the foot is in contact with the ground.
As just noted, the locomotion of a legged mobile robot becomes more unstable as the footfall impact gets larger. To say that a footfall impact is large means that its maximum value is high and the period over which it acts (its duration) is long. Since there is a strong tendency for the duration of a footfall impact to grow longer with increasing maximum value, reduction of the footfall impact impulse (the integral of the force over an interval of time) can be achieved by holding down the maximum value of the footfall impact. In particular, since a close relationship can be observed between the locomotion stability of a legged mobile robot and the maximum value of the footfall impact, one of the key aspects of stabilizing the locomotion of a legged mobile robot is that suppressing the maximum value of footfall impact is sufficient for achieving the stabilization. Moreover, since, as was pointed out earlier, a footfall impact can be deemed to be a force acting on the robot's center of gravity, locomotion can be stabilized by holding the maximum acceleration of the center of gravity to a low level.